A Review on Coupled Bulk Acoustic Wave MEMS Resonators.

With the introduction of the working principle of coupled resonators, the coupled bulk acoustic wave (BAW) Micro-Electro-Mechanical System (MEMS) resonators have been attracting much attention. In this paper, coupled BAW MEMS resonators are discussed, including the coupling theory, the actuation and sensing theory, the transduction mechanism, and the applications. BAW MEMS resonators normally exhibit two types of vibration modes: lateral (in-plane) modes and flexural (out-of-plane) modes. Compared to flexural modes, lateral modes exhibit a higher stiffness with a higher operating frequency, resulting in a lower internal loss. Also, the lateral mode has a higher Q factor, as the fluid damping imposes less influence on the in-plane motion. The coupled BAW MEMS resonators in these two vibration modes are investigated in this work and their applications for sensing, timing, and frequency reference are also presented.

Directed peptide amphiphile assembly using aqueous liquid crystal templates in magnetic fields.

An alignment technique based on the combination of magnetic fields and a liquid crystal (LC) template uses the advantages of both approaches: the magnetic fields offer non-contact methods that apply to all sample sizes and shapes, whilst the LC templates offer high susceptibilities. The combination introduces a route to control the spatial organization of materials with low intrinsic susceptibilities. We demonstrate that we can unidirectionally align one such material, peptide amphiphiles in water, on a centimeter scale at a tenfold lower magnetic field by using a lyotropic chromonic liquid crystal as a template. We can transform the aligned supramolecular assemblies into optically active π-conjugated polymers after photopolymerization. Lastly, by reducing the magnetic field strength needed for addressing these assemblies, we are able to create more complex structures by initiating self-assembly of our supramolecular materials under competing alignment forces between the magnetically induced alignment of the assemblies (with a positive diamagnetic anisotropy) and the elastic force dominated alignment of the template (with a negative diamagnetic anisotropy), which is directed orthogonally. Although the approach is still in its infancy and many critical parameters need optimization, we believe that it is a very promising technique to create tailor-made complex structures of (aqueous) functional soft matter.

Deciphering the Role of Water in Promoting the Optoelectronic Performance of Surface-Engineered Lead Halide Perovskite Nanocrystals.

Lead halide perovskites are promising candidates for high-performance light-emitting diodes (LEDs); however, their applicability is limited by their structural instability toward moisture. Although a deliberate addition of water to the precursor solution has recently been shown to improve the crystallinity and optical properties of perovskites, the corresponding thin films still do not exhibit a near-unity quantum yield. Herein, we report that the direct addition of a minute amount of water to post-treated formamidinium lead bromide (FAPbBr3) nanocrystals (NCs) substantially enhances the stability while achieving a 95% photoluminescence quantum yield in a NC thin film. We unveil the mechanism of how moisture assists in the formation of an additional NH4Br component. Alongside, we demonstrate the crucial role of moisture in assisting localized etching of the perovskite crystal, facilitating the partial incorporation of NH4+, which is key for improved performance under ambient conditions. Finally, as a proof-of-concept, the application of post-treated and water-treated perovskites is tested in LEDs, with the latter exhibiting a superior performance, offering opportunities toward commercial application in moisture-stable optoelectronics.

Origin of the polychromatic photoluminescence of zeolite confined Ag clusters: temperature- and co-cation-dependent luminescence.

Zeolite confined silver clusters (AgCLs) have attracted extensive attention due to their remarkable luminescent properties, but the elucidation of the underlying photophysical processes and especially the excited-state dynamics remains a challenge. Herein, we investigate the bright photoluminescence of AgCLs confined in Linde Type A zeolites (LTA) by systematically varying the temperature (298-77 K) and co-cation composition (Li/Na) and examining their respective influence on the steady-state and time-resolved photoluminescence. The observed polychromatic emission of the tetrahedral Ag4(H2O) n 2+ clusters ranges from orange to violet and three distinct emitting species are identified, corresponding to three long-lived triplet states populated consecutively and separated by a small energy barrier. These long-lived species are at the origin of the polychromatic luminescence with high photoluminescence quantum yields. Furthermore, the Li-content dependence of decay times points to the importance of guest-host-guest interactions in tuning the luminescent properties with a 43% decrease of the dominating decay time by increasing Li content. Based on our findings, a simplified model for the photophysical kinetics is proposed that identifies the excited-state processes. The results outlined here pave the way for a rational design of confined metal clusters in various frames and inspire the specified applications of Ag-zeolites.

Photothermal Suzuki Coupling Over a Metal Halide Perovskite/Pd Nanocube Composite Catalyst.

The development of improved catalysts capable of performing the Suzuki coupling reaction has attracted considerable attention. Recent findings have shown that the use of photoactive catalysts improves the performance, while the reaction mechanism and temperature-dependent performance of such systems are still under debate. Herein, we report Pd nanocubes/CsPbBr3 as an efficient catalyst for the photothermal Suzuki reaction. The photo-induced and thermal contribution to the overall catalytic performance has been investigated. Light controls the activity at temperatures around and below 30 °C, while thermal catalysis determines the reactivity at higher temperatures. The Pd/CsPbBr3 catalyst exhibits 11 times higher activity than pure CsPbBr3 at 30 °C due to reduced activation barrier and facilitated charge carrier dynamics. Furthermore, the alkoxide radicals (R-O-) for the Suzuki reaction are experimentally and theoretically confirmed, and photogenerated holes are proven to be crucial for cleaving C-B bonds of phenylboronic acids to drive the reaction. This work prescribes a general strategy to study photothermal catalysis and offers a mechanistic guideline for photothermal Suzuki reactions.

Simultaneous photocatalytic H2 generation and organic synthesis over crystalline-amorphous Pd nanocube decorated Cs3Bi2Br9.

Cs3Bi2Br9 decorated with crystalline-amorphous Pd nanocubes as cocatalysts is reported to photocatalytically coproduce ca. 1400 µmol h-1 g-1 of H2 and benzaldehyde from the selective benzyl alcohol oxidation. This route offers an alternative for photocatalytic H2 generation using metal halide perovskites under mild conditions.

Two-dimensional perovskites with alternating cations in the interlayer space for stable light-emitting diodes.

Lead halide perovskites have attracted tremendous attention in photovoltaics due to their impressive optoelectronic properties. However, the poor stability of perovskite-based devices remains a bottleneck for further commercial development. Two-dimensional perovskites have great potential in optoelectronic devices, as they are much more stable than their three-dimensional counterparts and rapidly catching up in performance. Herein, we demonstrate high-quality two-dimensional novel perovskite thin films with alternating cations in the interlayer space. This innovative perovskite provides highly stable semiconductor thin films for efficient near-infrared light-emitting diodes (LEDs). Highly efficient LEDs with tunable emission wavelengths from 680 to 770 nm along with excellent operational stability are demonstrated by varying the thickness of the interlayer spacer cation. Furthermore, the best-performing device exhibits an external quantum efficiency of 3.4% at a high current density (J) of 249 mA/cm2 and remains above 2.5% for a J up to 720 mA cm-2, leading to a high radiance of 77.5 W/Sr m2 when driven at 6 V. The same device also shows impressive operational stability, retaining almost 80% of its initial performance after operating at 20 mA/cm2 for 350 min. This work provides fundamental evidence that this novel alternating interlayer cation 2D perovskite can be a promising and stable photonic emitter.

Tuning the Structural and Optoelectronic Properties of Cs2 AgBiBr6 Double-Perovskite Single Crystals through Alkali-Metal Substitution.

Lead-free double perovskites have great potential as stable and nontoxic optoelectronic materials. Recently, Cs2 AgBiBr6 has emerged as a promising material, with suboptimal photon-to-charge carrier conversion efficiency, yet well suited for high-energy photon-detection applications. Here, the optoelectronic and structural properties of pure Cs2 AgBiBr6 and alkali-metal-substituted (Cs1- x Yx )2 AgBiBr6 (Y: Rb+ , K+ , Na+ ; x = 0.02) single crystals are investigated. Strikingly, alkali-substitution entails a tunability to the material system in its response to X-rays and structural properties that is most strongly revealed in Rb-substituted compounds whose X-ray sensitivity outperforms other double-perovskite-based devices reported. While the fundamental nature and magnitude of the bandgap remains unchanged, the alkali-substituted materials exhibit a threefold boost in their fundamental carrier recombination lifetime at room temperature. Moreover, an enhanced electron-acoustic phonon scattering is found compared to Cs2 AgBiBr6 . The study thus paves the way for employing cation substitution to tune the properties of double perovskites toward a new material platform for optoelectronics.

Tracking Structural Phase Transitions in Lead-Halide Perovskites by Means of Thermal Expansion.

The extraordinary properties of lead-halide perovskite materials have spurred intense research, as they have a realistic perspective to play an important role in future photovoltaic devices. It is known that these materials undergo a number of structural phase transitions as a function of temperature that markedly alter their optical and electronic properties. The precise phase transition temperature and exact crystal structure in each phase, however, are controversially discussed in the literature. The linear thermal expansion of single crystals of APbX3 (A = methylammonium (MA), formamidinium (FA); X = I, Br) below room temperature is measured using a high-resolution capacitive dilatometer to determine the phase transition temperatures. For δ-FAPbI3 , two wide regions of negative thermal expansion below 173 and 54 K, and a cascade of sharp transitions for FAPbBr3 that have not previously been reported are uncovered. Their respective crystal phases are identified via powder X-ray diffraction. Moreover, it is demonstrated that transport under steady-state illumination is considerably altered at the structural phase transition in the MA compounds. The results provide advanced insights into the evolution of the crystal structure with decreasing temperature that are essential to interpret the growing interest in investigating the electronic, optical, and photonic properties of lead-halide perovskite materials.

Imaging Heterogeneously Distributed Photo-Active Traps in Perovskite Single Crystals.

Organic-inorganic halide perovskites (OIHPs) have demonstrated outstanding energy conversion efficiency in solar cells and light-emitting devices. In spite of intensive developments in both materials and devices, electronic traps and defects that significantly affect their device properties remain under-investigated. Particularly, it remains challenging to identify and to resolve traps individually at the nanoscopic scale. Here, photo-active traps (PATs) are mapped over OIHP nanocrystal morphology of different crystallinity by means of correlative optical differential super-resolution localization microscopy (Δ-SRLM) and electron microscopy. Stochastic and monolithic photoluminescence intermittency due to individual PATs is observed on monocrystalline and polycrystalline OIHP nanocrystals. Δ-SRLM reveals a heterogeneous PAT distribution across nanocrystals and determines the PAT density to be 1.3 × 1014 and 8 × 1013 cm-3 for polycrystalline and for monocrystalline nanocrystals, respectively. The higher PAT density in polycrystalline nanocrystals is likely related to an increased defect density. Moreover, monocrystalline nanocrystals that are prepared in an oxygen- and moisture-free environment show a similar PAT density as that prepared at ambient conditions, excluding oxygen or moisture as chief causes of PATs. Hence, it is concluded that the PATs come from inherent structural defects in the material, which suggests that the PAT density can be reduced by improving crystalline quality of the material.

Giant Electron-Phonon Coupling and Deep Conduction Band Resonance in Metal Halide Double Perovskite.

The room-temperature charge carrier mobility and excitation-emission properties of metal halide perovskites are governed by their electronic band structures and intrinsic lattice phonon scattering mechanisms. Establishing how charge carriers interact within this scenario will have far-reaching consequences for developing high-efficiency materials for optoelectronic applications. Herein we evaluate the charge carrier scattering properties and conduction band environment of the double perovskite Cs2AgBiBr6 via a combinatorial approach; single crystal X-ray diffraction, optical excitation and temperature-dependent emission spectroscopy, resonant and nonresonant Raman scattering, further supported by first-principles calculations. We identify deep conduction band energy levels and that scattering from longitudinal optical phonons- via the Fröhlich interaction-dominates electron scattering at room temperature, manifesting within the nominally nonresonant Raman spectrum as multiphonon processes up to the fourth order. A Fröhlich coupling constant nearing 230 meV is inferred from a temperature-dependent emission line width analysis and is found to be extremely large compared to popular lead halide perovskites (between 40 and 60 meV), highlighting the fundamentally different nature of the two "single" and "double" perovskite materials branches.

Photophysical Pathways in Highly Sensitive Cs2 AgBiBr6 Double-Perovskite Single-Crystal X-Ray Detectors.

The sensitive detection of X-rays embodies an important research area, being motivated by a common desire to minimize the radiation doses required for detection. Among metal halide perovskites, the double-perovskite Cs2 AgBiBr6 system has emerged as a promising candidate for the detection of X-rays, capable of high X-ray stability and sensitivity (105 µC Gy-1 cm-2 ). Herein, the important photophysical pathways in single-crystal Cs2 AgBiBr6 are detailed at both room (RT) and liquid-nitrogen (LN2 T) temperatures, with emphasis made toward understanding the carrier dynamics that influence X-ray sensitivity. This study draws upon several optical probes and an RT excitation model is developed which is far from optimal, being plagued by a large trap density and fast free-carrier recombination pathways. Substantially improved operating conditions are revealed at 77 K, with a long fundamental carrier lifetime (>1.5 µs) and a marked depopulation of parasitic recombination pathways. The temperature dependence of a single-crystal Cs2 AgBiBr6 X-ray detecting device is characterized and a strong and monotonic enhancement to the X-ray sensitivity upon cooling is demonstrated, moving from 316 µC Gy-1 cm-2 at RT to 988 µC Gy-1 cm-2 near LN2 T. It is concluded that even modest cooling-via a Peltier device-will facilitate a substantial enhancement in device performance, ultimately lowering the radiation doses required.

Direct Laser Writing of δ- to α-Phase Transformation in Formamidinium Lead Iodide.

Organolead halide perovskites are increasingly considered for applications well beyond photovoltaics, for example, as the active regions within photonic devices. Herein, we report the direct laser writing (DLW: 458 nm cw-laser) of the formamidinium lead iodide (FAPbI3) yellow δ-phase into its high-temperature luminescent black α-phase, a remarkably easy and scalable approach that takes advantage of the material's susceptibility to transition under ambient conditions. Through the DLW of α-FAPbI3 tracks on δ-FAPbI3 single-crystal surfaces, the controlled and rapid microfabrication of highly luminescent structures exhibiting long-term phase stability is detailed, offering an avenue toward the prototyping of complex perovskite-based optical devices. The dynamics and kinetics of laser-induced δ- to α-phase transformations are investigated in situ by Raman microprobe analysis, as a function of irradiation power, time, temperature, and atmospheric conditions, revealing an interesting connection between oxygen intercalation at the surface and the δ- to α-phase transformation dynamics, an insight that will find application within the wider context of FAPbI3 thermal phase relations.

Confining Potential as a Function of Polymer Stiffness and Concentration in Entangled Polymer Solutions.

We directly track the tubelike motion of individual fluorescently labeled polymer molecules in a concentrated solution of unlabeled polymers. We use a single molecule wide-field fluorescence microscopy technique that is able to determine characteristic properties of the polymer dynamics, such as the confining potential, the tube diameter, and the Rouse time. The use of synthetic polymers allows us to investigate the confined motion of the polymer chains not only as a function of polymer concentration (mesh size) but also versus the persistence length of the matrix polymers. Although the polymers used have a persistence length much smaller than their contour length, our experimental results lead to a dependence of the tube diameter on both the mesh size and the persistence length, which follows the theoretically predicted relation for semiflexible chains.

Nanoscale Study of Polymer Dynamics.

The thermal motion of polymer chains in a crowded environment is anisotropic and highly confined. Whereas theoretical and experimental progress has been made, typically only indirect evidence of polymer dynamics is obtained either from scattering or mechanical response. Toward a complete understanding of the complicated polymer dynamics in crowded media such as biological cells, it is of great importance to unravel the role of heterogeneity and molecular individualism. In the present work, we investigate the dynamics of synthetic polymers and the tube-like motion of individual chains using time-resolved fluorescence microscopy. A single fluorescently labeled polymer molecule is observed in a sea of unlabeled polymers, giving access to not only the dynamics of the probe chain itself but also to that of the surrounding network. We demonstrate that it is possible to extract the characteristic time constants and length scales in one experiment, providing a detailed understanding of polymer dynamics at the single chain level. The quantitative agreement with bulk rheology measurements is promising for using local probes to study heterogeneity in complex, crowded systems.